In recent years, the use of implantable cardiac devices, such as pacemakers and defibrillators, has become increasingly common. Such devices are now used not only to treat and terminate cardiac arrhythmias, but also to provide valuable information to the clinician regarding the implanted device and its interactions with the patient.
Normal heart rhythm is termed normal sinus rhythm. The initiating impulse for a heart beat normally begins in an area high (towards the patient's head) in the right atrium called the sinus node. Electrical propagation spreads to the left atrium and down the right atrium simultaneously. Mechanical contraction follows electrical activation by a delay in the order of hundreds of milliseconds. At the base of the atria, the electrical signals converge on the atrio-ventricular (AV) node. The AV node conducts electrical impulses slowly from the atria to the ventricles, which provides blood from the atrial contraction time to fill the ventricles. When this relationship is appropriate and normal, as in normal sinus rhythm, the heart is said to be operating with atrio-ventricular (or AV) synchrony.
One type of arrhythmia known as a “junctional rhythm” results in a loss of proper atrio-ventricular synchrony. The term “junction” refers to the “AV junction”, also known as the “AV node”, and such rhythms are sometimes called nodal rhythms. During a junctional rhythm, the initiating impulse originates in the AV junction. Subsequently, the atria and ventricles are activated electrically and contract mechanically at nearly the same time, since retrograde conduction of the impulse from the AV node to the atria and antegrade conduction from the AV node to the ventricles takes about the same time. Near simultaneous contraction of the atria and ventricles does not allow transport of blood from the atria to the ventricles. The impact of this improper timing includes higher pressures in the atria, retrograde flow of blood into the veins that feed blood to the heart, and lack of ventricular filling. The latter results in a compromised hemodynamic status.
The AV node beats spontaneously at a rate faster than the sinus node or any other part of the heart in order to establish and maintain a junction rhythm. Generally, the portion of the heart which beats most rapidly establishes and controls the cardiac rhythm. If the heart rate is fast, it is called tachycardia. While tachycardia was initially described as greater than 100 beats per minute, it has come to be used in a more relative sense. A heart rate which is faster than appropriate for a given physiologic condition of a patient is now generally considered tachycardia although the rate may be less than the classical threshold of 100 beats per minute. Thus, a junctional rhythm is sometimes called junctional tachycardia.
The source of the junctional beat may also be referred to as an ectopic beat. A beat which originates from a location other than that which would be normal is said to be “ectopic”. As example, an ectopic atrial beat is one which does not originate from the sinus node. Beats which originate from the ventricles are ventricular ectopic beats. Junctional rhythms may also be called junctional ectopic tachycardia and abbreviated JET.
Symptoms associated with junctional rhythms are a result of the loss of AV synchrony resulting from aberrations of the normal conduction sequence of the heart. These symptoms may include shortness of breath, choking sensation, chest pain, fatigue, anxiety, dizziness, and confusion, all of which are generally considered to be signs of decreased cardiac output. In some instances, pulsations may manifest in the neck and abdomen of the patient due to the contraction of the atria on closed AV valves, which forces blood back into the venous system. Hypotension, pallor, cool extremities, diaphoresis, reduced urine output, jugular venous distension, ascites, and hepatojugular reflex can also be associated with junctional tachycardia.
Junctional tachycardia occurs frequently following surgical procedures to correct congenital heart defects (often children) and repair heart valves. These rhythms may occur because the patient's sinus node is slow or their AV node is fast. On an ambulatory basis, some patients experience junctional rhythms. Some are brief, infrequent episodes for which patients are asymptomatic, while others produce symptoms and/or are persistent. Since episodes occur infrequently, it is unlikely that such an episode will occur during examination of patients who have implanted devices such as a pacemaker or defibrillator. Junctional rhythms typically are not recognized during routine ambulatory ECG monitoring of patients with devices, as recognition of this arrhythmia is not reported by such systems. However, ambulatory recording with special monitors that incorporate telemetry from the implanted rhythm management device combined with algorithms designed to detect junctional rhythms has demonstrated that junction beats and junctional rhythms exist with a far higher frequency than previously understood in typical pacemaker or defibrillator (ICD) patient populations.
Dual chamber pacemakers and ICDs typically pace and sense the right atrium and right ventricle. Cardiac resynchronization therapy (CRT) devices pace and sense the right atrium, right ventricle and the left ventricle. One feature of such devices is the maintenance of atrio-ventricular synchrony. CRT devices restore and maintain inter ventricular synchrony, between left and right ventricles. These devices prevent bradycardia by maintaining a suitable atrial rate with atrial pacing and (A-V) ventricular synchrony with ventricular pacing. During AV block, ventricular pacing synchronized to spontaneous atrial beats or atrial pacing maintains proper A-V timing. Junctional rhythms, however, often render pacemakers inhibited.
If junctional rhythms produce atrial beats closely followed by ventricular beats, dual chamber pacemakers may sense the atrial beat but the closely following sensed ventricular beat causes the pacemaker to presume the rhythm is normal sinus rhythm, and no pacing will occur. If junctional rhythms produce atrial beats closely following ventricular beats, the action is essentially the same. Sensed ventricular beats cause the timing of the pacemaker to be reset and resume a new timing cycle for subsequent beats. Sequences of consecutive ventricular beats are generally recognized by pacemakers as premature ventricular contractions (PVCs) and recorded as such for later interrogation by clinicians.
Thus, implanted pacemakers are generally ineffective at treating junctional rhythms and physicians are presented with few if any clues in the patient's record as to the possible occurrence of such rhythms. If patients experience infrequent symptoms, their presentation during follow-up is with few if any clues about the possible occurrence of junctional rhythms as the explanation. For the patient in whom junctional rhythms are recognized, therapeutic options are quite limited. Raising the pacemaker rate allows the atrium to become faster than the junctional rhythm, but this option is nearly always unsatisfactory. While raising the rate of pacing when the patient presents in clinic for follow-up may allow restoration of AV synchrony and control of heart rate with the pacemaker at that time, activities of daily living will frequently cause the junctional rhythm to exceed the programmed rate of the pacemaker and loss of AV synchrony will again recur. Reprogramming the pacemaker to a higher rate that will overcome the junctional rhythm completely for all time is often at a rate which can not be comfortably or safely sustained.